What is the Difference Between .223 and 5.56 Rifle Cartridges and Chamberings?

Attempting to Clarify the Confusion between .223 Remington and 5.56 mm NATO Chambering

There seems to be a tremendous amount of confusion about the differences and similarities between rifles chambered in 223 Remington versus 5.56 mm NATO and the ammunition they require. While they are very similar cartridges and ammo suppliers tend to market them as synonymous, there are profound differences when it comes to considerations about chambering precision and safety. Here is a basic rundown between the two from somebody that has guns chambered in both, handloads for both, and shoots both (as well as the .223 Wylde chambering).

223 Remington cartridges are distinctly different from 5.56mm NATO cartridges although they look almost identical to the untrained eye.

223 Remington chambering is distinctly different from 5.56 NATO chambering as well.

5.56mm cartridges are loaded to a higher pressure (usually more powder) than a 223 Remington cartridge. This typically gives you more velocity with the 5.56mm round (barrel lengths being equal) than the 223 Remington. Both fire .224 caliber projectiles.

The difference between the 223 Remington and 5.56mm NATO chambers are in the throat area (also termed the "Leade" area). This is the portion of the barrel directly in front of the chamber where the rifling has been removed to allow space for the seated bullet. The throat in a 223 Remington chamber is typically .085" while the throat in the 5.56mm chamber is typically .162". The added space in the 5.56mm chamber compensates for the added pressure inherent in the 5.56mm NATO round.

The freebore area is the space that stands between the portion of the chamber machined to the fit the cartridge neck and the tapered beginning of the rifling in the barrel. This is also the distance that the bullet ogive (the point on the bullet that first contacts the rifling lands) has to travel before engaging the bore rifling.

The Leade is the full transition area from the end of the chamber to the full rifle land diameter. The beginning of the rifling is tapered to allow a gradual entry of the bullet to full rifling diameter (in our case, .224"). This encompasses the freebore area as well as the tapered rifling area.

Since the .223 Remington chambering dimensions are tighter than 5.56 NATO chambering, this can potentially create an over-pressure situation, exceeding SAAMI specifications, if you fire a 5.56mm NATO round in a 223 Remington chambered firearm. This overpressure situation is something that will surely damage your brass and potentially damage your firearms and you! This is the reason why you should not fire a 5.56mm NATO cartridge in a rifle chambered for 223 Remington.

The 5.56mm brass cases are a bit thicker in the walls and head than the 223 Rem. This is meant to add strength to the case for the higher pressures inherent in the 5.56mm round. While the added strength better contains the increased chamber pressure, this added thickness reduces the powder capacity and can further cause overpressure problems.

Since the 5.56 chambering is slightly more spacious, it is compatible with pressure profiles from either the 5.56 and/or .223. The larger chambering does not support the .223 cartridge as well (excess Leade dimensions leading to a larger bullet jump) and will cause a reduction in round to round consistency (which inevitably reduces round-to-round accuracy).

It is easy to identify 5.56mm from 223 Rem rounds. 5.56 mm NATO brass has a small "NATO insignia" stamped on the head of 5.56mm NATO ammo whereas the 223 Remington will have a "223 Rem" stamp. The little NATO icon looks like a circle with a + sign in the middle of it (the edges of the + touch the circle).

For reloaders, it is generally safe (you should inspect your brass for any defects) to reload 5.56mm NATO cases to 223 Remington load specifications although you must account for the added thickness in the 5.56mm case walls. It is advisable to always reduce your initial powder capacity and work up when developing a new load when using 5.56mm NATO stamped brass.

Most AR style rifles are 5.56mm NATO chambered unless specified differently from the manufacturer (their are exceptions such as early 1990's DPMS barrels). There have been some mismarked barrels in the past but manufacturers are, for the most part, abiding by proper chambering markings. If you happen to have an older barrel, you might contact the manufacturer to confirm the exact chambering of your rifle.

Some newer rifles are chambered with a .223 Wylde chambering. Think of this as a happy medium between .223 and 5.56 trying to satisfy both camps adequately. The .223 Wylde will support the firing of both 223 Remington and 5.56mm NATO rounds without exceeding SAAMI specs. This chambering also provides less bullet jump in 223 Rem leading to better round-to-round consistency which leads to improved accuracy.